1. Field of the Invention
This invention relates to an improvement in a load cell arrangement for precisely measuring loads applied to a measuring element contained within a protective case and more particularly, but not by way of limitation, to a load cell arrangement which precisely measures loads over a variable range which extends beyond the range of a conventional single range low load cell.
2. Description of the Prior Art
In the art of testing it is known to apply a variety of loads to a test subject and to measure the reaction, sometimes to destruction. There are known standard single range load cells which are devices capable of measuring loads applied to a measuring element contained within a protective case. The load cell measuring elements normally use strain gages for sensing the load applied, but other methods including induction, capacitance, magnetism and mechanical dial indicators are used. Load cells are used for measuring a single load or force applied in tension or compression very accurately. Such load cells are used extensively for measuring loads applied to various structures, weighing of many items, loads applied during production machining operations, wind tunnel testing, and numerous other applications.
The load cell maximum capacities vary from ounces to many tons. The accuracy of the measurements are based on the maximum load capacity of the load cell. The standard load cell accuracy for test applications is 0.10% to 0.50% of full load. To maintain accuracy through a large load range requires installing many different capacity load cells, in sequence which is very expensive and time consuming. As an example, at one-tenth of the maximum load the standard load cell would produce accuracies of 1.05% to 5.0% of the measured load. Thus, it is apparent that a need exists for a variable load cell arrangement that will reduce the number of load cells required for accurate data the entire desired test range and that will produce accurate data over the entire test load range, especially at the low load or near zero load portion of the range where it is impossible with the present day load cell designs.
It is believed that the present invention represents a major advance in load cell arrangements and satisfies the above objectives. An example of an application where the present invention may be used to advantage is seen in U.S. Pat. No. 4,920,791 which shows a wind tunnel model support and attitude control for wind tunnel testing. The present invention will allow the preload in the vanes of the model support system to be removed from the measuring elements, as a result only the variations in the vane loads due to the model air loads would be measured.